The primary goal of this research is to gain a better understanding of the physiological mechanisms underlying the hormonal control of behavior. The avian vocal control system provides a unique model system for studies of the neural mechanisms through which hormones influence behavior in higher vertebrates. Singing is controlled by a system of eight discrete, interconnected, hormone-sensitive brain nuclei. The vocal control system offers unique opportunities for studying the physiological bases of behavior, because it is a clearly-delineated, hormone-sensitive neural system with a clearly delineated behavioral function--it controls vocalizations, and to the best of our knowledge, nothing but vocalizations. Singing is stimulated by a complex interaction of androgens and estrogens. This research seeks to determine where these two classes of steroids exert their effects on the neural circuitry controlling singing and to assess the involvement of various neurotransmitter systems in mediating these steroid induced effects on behavior. Initially, the ability of behaviorally-active hormone treatments to affect neurotransmitter proves hormone sensitive, then the relative effects of androgens and estrogens on its function in the vocal control nuclei are determined. Finally, neuropharmacological experiments are carried out to determine if hormone-induced alterations in neurotransmitter function actually modulate behavior. A second line of research examines the interactive effects of hormones, neurotransmitters and experience on the development and maturation of this sexually- dimorphic neural system and its involvement in song learning. The first experiment seeks to determine neurotransmitter function at four critical points in development of the vocal control system and the process of song learning. Later experiments will manipulate endocrine or neurotransmitter function and determine the effects of these changes on learning or manipulate learning and determine how this affects neurotransmitter and endocrine function. The results of this research should help us better understand how steroid hormones and various neurotransmitter systems interact to modulate human brain function and behavior.